1. Field of the Invention
The present invention relates generally piezoelectric elements and specifically to methods and devices for controlling the operation of such elements.
2. Description of the Related Art
Most modern internal combustion engines utilize a fuel injection system to deliver atomized fuel to the engine by forcibly pumping the fuel through small orifices under high pressure. These fuel injection systems tend to be more precise and efficient than previously used carburetors. Typical fuel injectors utilized in these systems often utilize hydraulically, electromagnetically, or piezoelectrically actuated injector pins.
A piezoelectric element uses a material that changes dimensions when a voltage is applied across the element. When the voltage is removed, the piezoelectric element returns to its original dimensions. When used as actuators, many piezoelectric elements are stacked together to form larger piezoelectric elements or “piezoelectric stacks” to increase the displacement of the actuator. In a piezoelectrically actuated fuel injector, one or more of these piezoelectric elements or piezoelectric stacks are used to move a fuel injector pin for fuel metering into an internal combustion engine. Various spring-like structures are often used in conjunction with these devices to apply a return force and thus facilitate the return of the fuel injector pin to its “resting” position after the actuating energy is no longer applied to the piezoelectric stack.
Piezoelectric actuators are typically either voltage driven or current driven. Such actuators operate optimally at low actuation frequencies, but are limited to operating frequencies of no greater than about 100 hertz (Hz). At higher frequencies, the energy losses in the piezoelectric element cause self-heating of the piezoelectric stack. As the piezoelectric element's temperature increases, so does the piezoelectric element's capacitance. This capacitance increase in turn increases the charge required to operate the piezoelectric actuator. The increased charge requirement further increases heating and a phenomenon known as “thermal runaway” results. Thermal runaway involves the increasing capacitance resulting in increased current and increased heating which forms a positive feedback loop. This positive feedback loop ultimately raises the temperature of the piezoelectric actuator past the level where it can be optimally and/or safely operated.
An efficient method of control that allows the piezoelectric element to be efficiently and safely operated at higher frequencies without deleterious effects is thus needed.